The present invention relates to a double spindle synchronous driving apparatus which can be suitably employed as a driving apparatus for driving two spindles in highly accurate synchronism in a robot or the like.
For example, FIG. 1 shows a known robot for performing various operations at high positional accuracy relative to a workpiece. The known robot includes a Y-axis table 31, an X-axis table 32 which can be positioned at an arbitrary position in the direction of the Y-axis by the Y-axis table 31 and a working head 33 which ca be positioned at an arbitrary position in the direction of the X-axis by the X-axis table 32. A position recognizing means 34 and a working means 35 are provided on the working head 33 so as to be spaced a predetermined distance from each other in the direction of the X-axis.
However, in the known robot of the above mentioned arrangement, since the X-axis table 32 longitudinally extends over a long distance in the direction of the X-axis from the Y-axis table 31, the following problem arises. Namely, even if the X-axis table 32 is longitudinally inclined quite minutely, positioning error of the working head 33 in the direction of the Y-axis is produced when the working head 33 is displaced along the X-axis table 32 after the X-axis table 32 has been positioned in the direction of the Y-axis.
In order to solve the above mentioned problem, such an arrangement as shown in FIG. 4 may be employed. In the arrangement, a pair of Y-axis tables 1 and 2 are provided in parallel with each other so as to position opposite end portions of an X-axis table 7 in the direction of the Y-axis, respectively such that a working head 10 is positioned in the direction of the X-axis by the X-axis table 7. In the robot of the above described arrangement, if a pair of Y-axis motors 3 and 4 provided for the Y-axis tables 1 and 2 are not driven in highly accurate synchronism, the X-axis table 7 cannot be displaced stably.
Conventionally, an arrangement shown in FIG. 2 has been generally employed in a driving apparatus for synchronously driving two spindles. In the known arrangement, detection signals of pulse generators 43 and 44 for detecting rotational positions of motors 41 and 42 are applied to deviation counters 45 and 46 provided for motors 41 and 42, respectively, while a common position command signal is inputted to the deviation counter 45 and 46. A motor drive circuit 47 for the motor 41 is controlled on the basis of a deviation signal outputted from the deviation counter 45 and indicative of a deviation between the detection signal of the pulse generator 43 and the position command signal. Likewise, a motor drive circuit 48 for the motor 42 is controlled on the basis of a deviation signal outputted from the deviation counter 46 and indicative of a deviation between the detection signal of the pulse generator 44 and the position command signal. Meanwhile, the detection signals of the pulse generators 43 and 44 are also, respectively, inputted to frequency/voltage (FV) converters 49 and 50 and output signals of the FV converters 49 and 50 are applied to the motor drive circuits 47 and 48, respectively so as to perform speed compensation of the motors 41 and 42.
However, in the prior art double spindle synchronous driving apparatus of the arrangement shown in FIG. 2, since the two motors 41 and 42 are controlled in response to the position command independently of each other such that the deviation between the commanded position and the detected position of each of the motors 41 and 42 assumes zero, such an inconvenience is incurred. Namely, even in the case where, for example, a large delay happens between the commanded position and the detected position of only one of the motors 41 and 42 in a fixed speed driving state for some reason or other, the other of the motors 41 and 42 will maintain its predetermined operational state. Therefore, asynchronism between the motors 41 and 42 is not eliminated quickly and thus, it is difficult to achieve highly accurate synchronous driving.